A methodology for the heat of immersion as a measure of wettability of mineral mixtures

Magudu, Anam

A methodology for the heat of immersion as a measure of wettability of mineral mixtures

Thesis / Dissertation

2023

Abstract

The measure of the extent to which a mineral interacts with water is called wettability and this is important in flotation processes. This is because the interactions between solid particles and liquid molecules (water) are important in understanding the flotation mechanism and achieving high recoveries. Contact angles and work of adhesion can be used to determine the physical properties of a given solid-liquid system, but there are drawbacks to these techniques. The advancement of microcalorimetry instrumentation has led to the use of heat of immersion to determine the surface wettability of solid surfaces. Several calorimetric studies have proven that the heat of immersion can be used to determine the surface wettability of minerals. Previous research within the Centre for Minerals Research (CMR) has shown that the heat of immersion can provide a reliable measure of mineral surface wettability when it is measured by precision solution calorimetry. However, this was done only for single mineral systems and its application to real ores has not been investigated in depth. In this study, the heat of immersion as a measure of wettability is applied to a simple binary mineral mixture representative of a real ore. The binary mineral mixture consists of a hydrophobic sulphide mineral, galena and a hydrophilic silicate mineral, albite. The results in this study have shown that the heat of immersion measurements present challenges such as an unexpected endothermic response. This endothermic response is attributed to the dissolution of the mineral in water. This dissolution is due to the surface ions on the mineral being exposed to the wetting liquid. In order to predict flotation response through measuring wettability, the aim is to measure only the heat of wetting, which is an exothermic response. Therefore, the dissolution process needs to be suppressed. Alternative techniques such as a solution saturated with the mineral sample, using organic liquids as the wetting liquids, and pre-coating of the mineral particles with collector were explored. From the various approaches explored to suppress the dissolution, it was observed that the saturated solution approach was an effective technique for certain minerals such as albite but was ineffective at suppressing the dissolution process across a range of mineral types. It is, therefore, an ineffective technique for exploring the heat of immersion of binary mineral mixtures. Secondly, it was observed that the collector coating approach was effective for suppressing dissolution at surface coverages above 75%. The collector coating approach is not feasible for conducting the heat of immersion measurements for the binary mineral mixtures because it only successfully suppresses dissolution at excess surface coverages that are not necessarily those at which one would choose to do the experimental work. Additionally, collector coating does not allow for the natural wettability of the uncoated minerals to be measured. Thirdly, hexane was found as a good wetting liquid for suppressing dissolution but there were some experimental difficulties that led to this liquid not being used for the binary mineral mixtures. These experimental difficulties include a premature immersion of the mineral into the wetting liquid due to the beeswax used to seal the ampoule dissolving in the hexane. Finally, hexanol was found to be a good wetting liquid in suppressing dissolution, had no associated experimental difficulties and was able to distinguish relative hydrophobicities between different mineral surfaces. It can, therefore, be used as an effective wetting liquid for mineral dissolution suppression and hydrophobicity determination. Preliminary experimental work into the feasibility of using a binary mineral mixture as a simple model ore system was performed. A linear relationship was found between the heat of immersion and the fraction of pure mineral A in a binary A + B mineral mixture. The heat of immersion could be presented in various ways depending on what data is required and desired. The surface area fraction or mass composition can be used to create the linear relationship between the heat of immersion and the composition of the binary mineral mixture. It was shown that there is a linear relationship between the heat of immersion and mass composition or surface area fraction of the binary mineral mixture. From this linear relationship, the heat of immersion of the pure minerals comprising the mixture can be extrapolated. The linear relationship based on composition provides a simple and convenient way to estimate hydrophobicity of a floatable mineral in an ore where only the mass and mineral composition of the sample is known. This could be used in flotation modelling, where valuable mineral floatability is a required input parameter. To determine the relative hydrophobicity of a binary mineral mixture in hexanol where the mass composition is unknown, the heat of immersion or heat released by the binary mineral mixture is measured and this is correlated with the mixture's mineral weight composition. This linear relationship can then be extrapolated to zero and 100% respectively to obtain the heat of immersion of the pure minerals. These values can be read off a calibration curve such as that obtained by Taguta et al. (2018) to obtain a flotation rate constant.

Keywords

Engineering

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